1. Field of the Invention
This invention relates to a method of fabricating a ball grid array for use in conjunction with semiconductor devices.
2. Brief Description of the Prior Art
Plastic ball grid array packages are well known in the prior art. In general, such packages include a printed wiring board having a top surface to which a semiconductor die is secured and a reverse surface. Electrically conductive traces are positioned on the printed wiring board top surface and are spaced from the die with wire bonds extending from the pads on the die to the traces. Vias extend through the printed wiring board and couple traces on the printed wiring board with traces on the opposing or reverse surface of the printed wiring board. A potting agent or plastic encapsulant is disposed over the die, wire bond and a portion of the trace on the top surface. Cleaning then takes place to remove contamination so that solder balls can be attached to the leads or traces by reflow soldering. The solder ball is formed in standard manner and secured to each trace on the reverse side of the printed wiring board, preferably over the via. Testing and inspection then takes place. The package is soldered to a circuit in standard manner by proper positioning and heating and/or soldering of the solder ball in standard manner. An advantage provided by use of the balls is that the balls attract the solder by a wicking action due to their circular shape and thereby provide a highly reliable and relatively strong solder connection.
A problem with the above described approach is that it does not utilize standard semiconductor fabrication processes and therefore requires an entirely separate process and equipment for such fabrication. A further problem with this prior art plastic ball grid array package is that the procedure requires a sizable number of costly fabrication steps, resulting in relatively high fabrication costs. It follows that a procedure for fabrication of the ball grid array is required which will result in lower costs.
In accordance with the present invention, a ball grid array assembly is provided which is fabricated at far lower cost than that of the prior art using existing QFP processes and equipment.
Briefly, the balls, which are formed of an electrically conductive material, preferably copper or steel, and preferably capable of accepting a nickel plating, are initially attached to standard QFP lead frame leads (it being understood that lead traces on the printed wiring board or package can be substituted for the lead frame leads herein) in standard manner, such as, for example, by welding, brazing or soldering. The balls and lead frame are then plated, generally with a layer of nickel followed by a layer of palladium, prior to commencement of fabrication of the package. This preplates the balls as well as the lead frame or traces before the assembly process. The die attach step then takes place on the lead frame with subsequent wire bonding of wire between die pads on the die and the lead frame leads in the standard manner. Then encapsulation takes place by molding in a mold cavity with the balls generally protruding from the encapsulant. The flash over the balls, if any, is then removed by a simple abrading or etching procedure which insures that the lead frame/ball plating initially placed over the lead frame and balls is retained. Package singulation, testing and inspection then takes place. It can be seen that standard semiconductor leadframe processing procedures as opposed to those procedures normally used for fabrication of ball grid arrays have been used to obtain the final ball grid array package in accordance with the present invention with the only change required being that of securing the balls to the lead frame lead finger before leadframe plating and removal of any mold flash over the balls. Also, as will be explained hereinbelow, a slightly modified mold cavity is required for performance of the molding operation.